News Release Number: STScI-2003-18

The Secret Lives of Galaxies Unveiled in Deep Survey

The full news release story:

Two of NASA's Great Observatories, bolstered by the largest ground-based
telescopes around the world, are beginning to harvest new clues to the
origin and evolution of the universe's largest building block, the
galaxies. It's a bit like finding a family scrapbook containing
snapshots that capture the lives of family members from infancy through
adolescence to adulthood.

"This is the first time that the cosmic tale of how galaxies build
themselves has been traced reliably to such early times in the
universe's life," says Mauro Giavalisco, head of the Hubble Space
Telescope portion of the survey, and research astronomer at the Space
Telescope Science Institute (STScI) in Baltimore, Md.

So far, the Hubble Space Telescope has joined forces with the Chandra
X-ray Observatory to survey a relatively broad swath of sky encompassing
tens of thousands of galaxies stretching far back into time. (This
unprecedented survey will soon be joined by the Space Infrared Telescope
Facility (SIRTF), which is to be launched in August 2003).

Called the Great Observatories Origins Deep Survey (GOODS), astronomers
are studying galaxy formation and evolution over a wide range of
distances and ages. The project is tracing the assembly history of
galaxies, the evolution of their stellar populations, and the gusher of
energy from star formation and active nuclei powered by immense black
holes.

In preliminary results soon to be published in the Astrophysical Journal,
Hubble astronomers report that the sizes of galaxies clearly increase
continuously from the time the universe was about 1 billion years old to
an age of 6 billion years. (This is approximately at half the current age
of the universe, 13.7 billion years.) GOODS astronomers also find that
the star birth rate rose mildly (by about a factor 3) between the time the
universe was about one billion years old and 1.5 billion years old, and
remained high until about 7 billion years ago, when it quickly dropped to
one-tenth the earlier "baby boomer" rate. This is further evidence that
major galaxy building trailed off when the universe was about half its
current age.

This increase in galaxy size is consistent with "bottom-up" models,
where galaxies grow hierarchically, through mergers and accretion of
smaller satellite galaxies. This is also consistent with the idea that
the sizes of galaxies match hand-in-glove to a certain fraction of the
sizes of their dark-matter halos. Dark matter is an invisible form of
mass that comprises most of the matter in the universe. The theory is
that dark matter essentially pooled into gravitational "puddles" in the
early universe that then collected normal gas that quickly contracted to
build star clusters and small galaxies. These dwarf galaxies merged
piece-by-piece over billions of years to build the immense spiral and
elliptical galaxies we see today.

The Chandra observations amounted to a "high-energy core sample" of the
early universe, allowing us to "study the history of black holes over
almost the entire age of the universe," said Niel Brandt of Penn State
University, a co-investigator on the Chandra GOODS team, who studied the
X-ray results. One of the fascinating findings in this deepest X-ray
image ever taken is the discovery of mysterious black holes, which have
no optical counterparts.

"We found seven mysterious sources that are completely invisible in the
optical with Hubble," said Anton Koekemoer of the STScI, a
co-investigator on both the Hubble and Chandra GOODS teams, who compared
the X-ray and optical reslts. "Either they are the most distant black
holes ever detected, or they are less distant black holes that are the
most dust enshrouded known, a surprising result as well."

When comparing the Hubble and Chandra fields, astronomers also found
that active black holes in distant, relatively small galaxies were
rarer than expected. This may be due to the effects of early
generations of massive stars that exploded as supernovae, evacuating
galactic gas and thus reducing the supply of gas needed to feed a
supermassive black hole.

These and other results from the GOODS project will be published in a
special issue of the Astrophysical Journal Letters, entirely devoted to
the team's results. The Chandra results are found in papers led by
Koekemoer and Stefano Cristiani of the Trieste Astronomical Observatory.
Hubble's findings came from papers led by Giavalisco, Mark Dickinson,
and Harry Ferguson of the STScI.